Acorn Woodpecker

Melanerpes formicivorus

Order:
Piciformes
Family:
Picidae
Sections

Distribution, Migration, and Habitat

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Figure 1. Distribution of the Acorn Woodpecker in North and Middle America.

This species also occurs in northern South America. Only a few populations of this species are migratory; see text for details.

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eBird range map for Acorn Woodpecker

Generated from eBird observations (Year-Round, 1900-present)

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Figure 2. Annual cycle of breeding, molt, and migration of the Acorn Woodpecker in central coastal California.

Thick lines show peak activity, thin lines off-peak. Although most individuals breeding in this and other populations do so in spring, some breed in the autumn as well. See text for details.

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Example of Acorn Woodpecker habitat: Hastings Reservation, Monterey County, California.

Mostly blue oak savannah.

© Walt Koenig , California , United States , 9 September 2007
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Example of Acorn Woodpecker habitat: Hastings Reservation, Monterey County, California.

Valley and blue oak woodland.

© Walt Koenig , California , United States , 23 April 2010
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Example of Acorn Woodpecker habitat: Hastings Reservation, Monterey County, California.
© Walt Koenig , California , United States , 11 January 2011
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Example of Acorn Woodpecker habitat: Belize, Belize.
© Yousif Attia , Belize , Belize , 13 November 2015
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Example of Acorn Woodpecker habitat: Cartago, Costa Rica.
© Alex Lamoreaux , Cartago , Costa Rica , 3 January 2019

Distribution in the Americas

Year-Round Range

Figure 1. Resident throughout interior valleys and hills of western Oregon, in Siskiyou Mountains of Curry and Coos counties in extreme southwestern Oregon, and an isolated population in The Dalles, Wasco County, in north-central Oregon (32); at least one population has crossed the Columbia River in the vicinity of Lyle, Washington. In California, resident in the north throughout Klamath Mountain region and Cascade Mountains east to southwestern Modoc County, and south along the western slopes of the Sierra Nevada to the mountains encircling the south end of the San Joaquin Valley, and south (west of desert areas) almost to the Mexican border. Absent from many coastal lowlands from Del Norte County to Mendicino County in the north and from Santa Barbara to San Diego County in the south, and absent from the San Joaquin and adjacent Salinas valleys, as well as from all eastern deserts (33, 34). Small, isolated populations reside in mountains northeast of Honey Lake in Lassen County, the mountains of Plumas and Sierra counties in northeastern California (34), and Inyo County. Isolated populations also reside in extreme northeastern and Cape area of extreme southern Baja California (35). Resident throughout mountains of much of Arizona from the southeastern through northwestern part of state (36, 37), and New Mexico (excluding northern and eastern areas; 38). In western Texas, common resident in Chisos, Davis, and Del Norte mountains, but uncommon and local in the Guadalupe Mountains and the Sierra Diablo; a small remnant population occurs in Bandera, Kerr, and Real counties in the central Edwards Plateau (39). Resident in Mexico on the Pacific slope from eastern Sonora and western Chihuahua and on the Atlantic slope from Coahuila and from Guanajuato in the interior south through central Chiapas, southern Guatemala, northern El Salvador, and nearly throughout Honduras to northwestern and northeastern Nicaragua (35, 40, 41). Also occurs disjunctly in northern Guatemala, eastern El Salvador, and southern Tamaulipas, Mexico (35), and Mountain Pine Ridge and coastal plain of Belize (42). Resident from the Cordillera Central and Cordillera de Talamanca of Costa Rica (timberline down to 1,500 m; 43) to the highlands of western Chiriquí, Panama (mostly above 1,200 m; 44). Major peripherally isolated populations also occur in the Andean slopes of northern and western Colombia (4, 45, 46).

Distribution Outside the Americas

No known records.

Nature of Migration

Most populations are resident. In areas where there are large seasonal fluctuations in insects and other foods, year-round residency is dependent on the birds' ability to store sufficient acorn mast to provide food throughout the winter. Groups that exhaust their stores often abandon their territories and wander off in search of alternative food. Depending on the magnitude and extent of the crop failure, birds that abandon their territories may find space and acorns close by (47) or be forced to leave the immediate vicinity but remain close enough to return the following spring (48, 49). Widespread mast-crop failure can lead to permanent disappearance of a large proportion of the population and may lead to a "flight year" in which birds are found in habitats that are normally unsuitable, including deserts and open grasslands (36). Vagrancy, due to acorn crop failures or long-distance dispersal events, are relatively common leading to records up to about 200 km away from known resident populations (50).

At least 1 population, located near the Huachuca Mountains in southeastern Arizona, regularly migrates (51). Breeding territories at this site are established in open oak woodlands. There are no granaries, and acorns are stored in natural holes and cracks in the bark. In most years, all stored mast is quickly exhausted and the birds then leave until the following spring, presumably migrating south to the extensive oak forests (encinal) of the Sierra Madre of Mexico. In spring, adults return to the site independently. Most, but not all, reoccupy their previously held territory. Among the migrants, birds reproduce in single male-female pairs without nonbreeding helpers, and cooperative breeding is absent. All young disperse in the fall, and there is no natal philopatry. However, in the rare years when birds can store sufficient mast in natural locations to provide food throughout the winter, birds remain resident and may even may breed in groups with ≥ 2 males.

Habitat in Breeding Range

Oak (encinal) and pine–oak (PinusQuercus) woodlands; also along riparian corridors, and in Douglas-fir (Pseudotsuga spp.), redwood (Sequoia sempervirens), and montane hardwood forests in the Neotropics as long as oaks are present or available nearby. Found at sea level in southern California, but more generally in mountains up to the distributional limit of oaks; occurs from 1,400–3,300 m in Colombia (45). Also common in urban parks and suburban areas whenever oaks are present and trees or other natural or human-made structures are available for acorn storage. Analyses of Christmas Bird Count data by Bock and Bock (52) and Koenig and Haydock (53) have found that along the Pacific Coast of North America the distributional limit of this species is generally set not by the limit of oaks, but by sites where oak diversity drops to a single species, presumably due to the lower probability of populations experiencing a total acorn crop failure. Acorn Woodpecker densities increase with increasing abundance of oaks while population variability decreases with increasing diversity of oaks. Comparable patterns were not found in the American Southwest, where densities are generally much lower than along the Pacific Coast.

At least a few populations, including the northern and southern range limits of this species, extend beyond areas with multiple oak species, however. They are common, for example, on the Monterey Peninsula of central coastal California, within the range of only one native oak species (the coast live oak Quercus agrifolia). Similarly, in the Pacific Northwest, birds have colonized southern Washington and north-central Oregon within the range of only one species of oak (the Garry oak Quercus garryana). They have thus far been recorded only accidentally further north despite the range of Garry oaks extending into British Columbia. On the southern end of its range, several isolated populations live in Colombia dependent on the Colombian oak (Q. humboldtii). Work by Freeman and Mason (46) confirmed that the distribution in the Northern Andes was overpredicted by climate-only models and better predicted by distribution models incorporating the presence of the Colombian oak, thus supporting the hypothesis that oaks limit the geographic range of this species in South America.

Habitat in the Overwintering Range

For resident populations, same as breeding. For migrant populations, see Nature of Migration.

Historical Changes to the Distribution

Adapts well to suburban conditions, so declines in distribution due to human activities are primarily due to habitat degradation and elimination of snags (standing dead trees), especially those used for acorn storage (see Conservation and Management: Management). A good colonizer, regularly found outside its normal range sometimes far from breeding habitat, although usually within 200 km of known populations, including on the Farallon Islands, 30 km off the coast of San Francisco, California (54, 55) and near Independence, Inyo County, California (34). Such vagrants have established several new populations and have extended the range of this species significantly within historic times, including both Santa Cruz and Santa Rosa Islands, 30 km off the coast of southern California (56, 57, 58), northward into Oregon (59), southern Washington across the Columbia River, and southwestern Colorado (K. Kaufman, personal communication), and across the Sierra Nevada near Susanville, Lassen County, California (34), the last apparently occurring subsequent to Grinnell and Miller's (60) survey.

Fossil History

Reported from only one late Pleistocene (late Rancholabrean, North American Land Mammal Age, < 400,000 years before present) locality, which is Shelter Cave, New Mexico (61).

Recommended Citation

Koenig, W. D., E. L. Walters, P. B. Stacey, M. T. Stanback, and R. L. Mumme (2019). Acorn Woodpecker (Melanerpes formicivorus), version 2.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.acowoo.02